Fluoropolymers have been used in a variety of applications because of several desirable properties such as heat resistance, chemical resistance, weatherability, and UV-stability. Fluoropolymers include homo and co-polymers of a gaseous fluorinated olefin such as tetrafluoroethylene (TFE), chlorotrifluoroethylene (CTFE) and/or vinylidene fluoride (VDF) with one or more gaseous or liquid comonomers such as hexafluoropropylene (HFP) or perfluorovinyl ethers (PVE) or non-fluorinated olefins such as ethylene (E) and propylene (P).
Fluoropolymers include melt-processable and non-melt-processable polymers. For example, polytetrafluoroethylene and copolymers of tetrafluoroethylene with small amounts (e.g. not more than 0.5% by weight) of a comonomer are generally not melt-processable with conventional equipment because of high molecular weight and high melt viscosity. Accordingly, for these non-melt-processable fluoropolymers, special processing techniques have been developed for forming these fluoropolymers into desired articles and shapes.
Melt-processable thermoplastic fluoropolymers are also known and these can be obtained from various combinations of fluorinated and/or non-fluorinated monomers. As they are melt-processable, they can be processed with conventional equipment. Melt-processable thermoplastic fluoropolymers include generally amorphous fluoropolymers and fluoropolymers that have substantial crystallinity. Fluoropolymers that are generally amorphous are typically used to make fluoroelastomers by curing or vulcanizing the fluoropolymer. Although, the elastomeric properties generally are obtained after curing, the fluoropolymers used for making the fluoroelastomer are often also called fluoroelastomer. Melt-processable thermoplastic fluoropolymers that have substantial crystallinity and that accordingly have a clearly detectable and prominent melting point are known in the art as fluorothermoplasts or thermoplastic fluoropolymers.
The rate of extrusion of fluorothermoplast is limited to the speed at which the polymer melt undergoes melt fracture. This may be important in thermoforming processes such as wire and cable extrusion, film extrusion, blown film extrusion and injection molding. If the rate of extrusion exceeds the rate at which melt fracture occurs (known as critical shear rate), an undesired rough surface of the extruded article is obtained. Bimodal fluorothermoplasts, for instance, THV and FEP, have been used in attempts to increase the extrusion speed by a substantially broadening the molecular weight distribution (MWD) of the extruded polymer, thereby increasing the critical shear rate. The gain in critical shear rate, however, is typically accompanied by weaker overall mechanical properties such as flex life.
The process rate of fluorothermoplasts in wire and cable extrusion can also be increased by using an extrusion die with a relatively large orifice and then drawing the extruded melt to the desired final diameter. The melt draw is commonly characterized by the draw down ratio calculated as the ratio of the cross-sectional area of the die opening to the ratio of the cross-sectional area of the finished extrudate. Typical draw down ratios of wire and cable extrusions are on the order of about 100. This melt draw, however, includes that high elongational rates, which characterize the rate of the melt draw, are usually in the order of 1 to 1000 l/s. The polymer melt should exhibit a sufficiently high elongational viscosity. Otherwise the cone stability of the polymer melt in the extrusion will be insufficient, which results in undesired diameter variations of the extruded article as well as frequent cone-breaks.
Further attempts to increase the cone stability have included the use of perfluoro vinylethers (PVE), for example with FEP. PVEs have been added (as comonomers in fluorothermoplasts) in an attempt to retain mechanical properties while increasing the processing speed of the fluorothermoplasts. But, the additional incorporation of PVEs into fluorothermoplasts increases the manufacturing costs, which may not be desired. Furthermore, the formation of die deposits (“die drool”) may occur, particularly with a broad MWD of the fluorothermoplast. In fast extrusion procedures, such as wire & cable insulation, large accumulation of die deposits separate from the die and may cause break-off of the melt cone (“cone-break”) and thus interruption of the production.